1. Field of the Invention
This invention relates to a fastener for securing a bone plate to surrounding bone, for example when replacing a portion of the cranial vault removed during a craniotomy for a neurosurgical procedure. The invention also relates to an instrument for securing two threaded members to each other. The instrument may be used during neurosurgical procedures, or as a mechanical tool in many other contexts.
2. General Discussion of the Background
A craniotomy is a surgical procedure in which a portion of the cranial vault is removed or folded back in a flap to permit surgical access to the cranial contents (such as the brain). To perform the craniotomy, several burr holes are drilled through the skull. The number and position of these holes varies depending on the shape of bone to be removed. For example, three burr holes are drilled at corner points if a triangular bone flap is desired. The burr holes are then connected by osteotomy cuts, for example using a Gigli flexible saw which is passed internally between the burr holes. The saw is then oscillated back and forth to cut the skull along a line of separation (defined by the connecting osteotomies) connecting adjoining burr holes. The bone cover is subsequently lifted off the underlying dura mater to expose the brain. The bone cover may either be completely removed from the surgical site, or folded back in a flap along an uncut edge of the flap.
After completion of the operation, the bone cover must again be fixed in its original position to protect the underlying brain. Conventional craniotomy closure requires that holes be drilled in the bone plate and surrounding cranium along the osteotomy lines, and stainless steel wire or silk sutures are then passed through the holes to retain the plate in place. Unfortunately, drilling the holes is time consuming and potentially dangerous, because of the risk of introducing infection. The sutures are also unstable and prone to breakage. Suture instability or breakage can lead to dangerous movements of the cranial plate against the brain, with pathologic sequelae similar to a depressed skull fracture. The sutures closing the osteotomy lines are not aesthetically pleasing, because they can leave irregularities in the overlying surface of the face or scalp. This irregularity is particularly unsightly if the surface of the bone plate is not held substantially co-planar with the surrounding bone.
Various fixtures have previously been proposed for securing the bone cover to the surrounding cranium. U.S. Pat. No. 5,201,737 discloses a flexible plate having a plurality of vanes with holes for receiving bone screws. The plate is placed over a cranial burr hole and adjoining osteotomy lines to provide external fixation of the bone cover to the surrounding cranium.
Other external bone plates are shown in U.S. Pat. No. 4,651,724; U.S. Pat. No. 4,923,471; U.S. Pat. No. 5,139,497 and U.S. Pat. No. 5,372,498. All of these plates are designed for external application to fractured bones, and require placement of a plurality of screws through the plates. As with the plate in U.S. Pat. No. 5,201,737 discussed above, placement of multiple screws through the plates is time consuming, predisposes to catastrophic infection, and is difficult to remove once in place.
A variety of fixation devices are also known for fusing fractured bones. An example of such a device is U.S. Pat. No. 2,511,051, in which an externally threaded stud screws into an internally threaded shank. Movement of the stud into the shank is guided by an hexagonal wrench that is inserted through the shank into a countersunk receptacle on the tip of the threaded stud.
U.S. Pat. No. 3,875,936 shows an attachment for replacing a trochanteric head to the femur by providing a barbed shear washer between the femur and trochanteric head. U.S. Pat. No. 5,098,433 uses a winged compression bolt for fusing fractured bones. U.S. Pat. No. 5,196,016 and U.S. Pat. No. 5,433,719 discloses fixation pins or screws for retaining bone fragments against one another.
In spite of the use of a variety of fasteners in orthopedic and neurosurgical procedures, improved techniques are still being sought to secure a cranial cover to the surrounding cranium following a craniotomy. Improved methods and devices for securing the fasteners to the skull are also disclosed.
Accordingly, it is an object of the present invention to provide a fastener that is especially suitable for closure of craniotomies.
Yet another object is to provide such a fastener that can be quickly and efficiently installed, and which is capable of easy removal in the event that subsequent intracranial access is required for another neurosurgical procedure.
Yet another object of the invention is to provide such a fastener that avoids the aesthetic drawbacks of prior fasteners, such as large indentations in skin overlying the craniotomy.
It is another object of the invention to provide such a fastener, and an instrument for manipulating the fastener, that allows it to be easily inserted and removed, yet which provides a potentially permanent and reliable fixation of the cranial cover, thereby avoiding the potentially catastrophic neurological consequences that can result from dislodgement or depression of the cranial cover.
Finally, it is an object of the invention to provide an instrument that is useful for engaging first and second members to each other, particularly when the engagement must occur on opposite surfaces, and one of the surfaces is not easily accessible.